According to Kinetic theory, the liquid particles encompass lower average Kinetic energy than the gas and movement is an intermediate among those of solid and gas. The liquid particles are much broader apart and are held via weaker forces of attraction than in the solid state.
We are familiar from the heating curve that, when solid matter is heated it changes to the liquid form at the melting point. The cooling of vapor (gas) as well yields in liquid formation, the process is termed as condensation. The liquid state is thus an intermediate state between the solid and gas states. Liquids are fluids, such as gases. They flow and can be poured from one vessel to the other. Dissimilar to gases they encompass fixed volumes and are uncompressible.
The Vapor Pressure of Liquids:
We are familiar that a liquid sample comprises of molecules with varying Kinetic energies. A few particles encompass very high energies whereas some encompass low energies. Now take a liquid sample positioned in a closed vessel or container in the laboratory. A few energetic particles which are close to the liquid surface will vaporize and comprise a vapor pressure above the liquid. This pressure builds up till the space above the liquid is saturated by vapor. If this occurs, then a dynamic equilibrium is set up among the liquid particles in the vapor and the liquid phase.
The pressure of the vapor beneath this equilibrium condition is known as the saturated vapor pressure (SVP) of the liquid. This is the maximum vapor pressure of the liquid at that temperature. The SVP is not dependent on the liquid volume in the vessel or container and it increases by increasing temperature as shown in the figure given below:
Liquids having strong intermolecular forces usually have low saturated vapor pressure than those having weak intermolecular forces.
Fig: Change of saturated vapor pressure with temperature
Boiling and Boiling Point:
The atmosphere around us applies a pressure on all objects on the surface of earth. The pressure of the atmosphere is known as the atmospheric pressure. Its value reduces with altitude (that is, height above the sea level). This means that the atmospheric pressure on top of a high mountain is less than at the base.
A heated liquid is as well acted on via the atmospheric pressure however as it is heated, its saturated vapor pressure increases, opposing the downward effect of the atmospheric pressure. At a specific temperature characteristic of the liquid, the saturated vapor pressure becomes equivalent to the atmospheric pressure. At this temperature vaporization becomes most fast and bubbles of vapor form even from the interior of the liquid. This is known as boiling and the temperature is termed as the boiling point. By statement, the boiling point is the temperature at which the saturated vapor pressure of the liquid equivalents the prevailing atmospheric pressure.
The definition above doesn't allow for a reference point of measurement as the atmospheric pressure is not a constant at each and every point on the surface of earth. This is due to the reason that the atmosphere encompasses many gases comprising water vapor. It is therefore a mixture and depending on the place in the earth and the conditions, its composition is not for all time similar. The normal (or standard) boiling point of a liquid is the temperature at which its saturated vapor pressure equals 760 mm-Hg is known as Φ atmosphere pressure. The normal boiling point of water is 100°C however due to the atmospheric pressure sometimes; it is slightly less than 760 mm Hg water boils at less than 100°C. Measurements describe that water boils at 98°C in many places.
The saturated vapor pressure of acetone at 20 °C is much higher than that of water at similar temperature that liquid will encompass the higher boiling point, water or acetone?
Boiling Point - A Criterion of Purity
Most of the substances in nature exist in the impure state. For example, polluted water poses danger to the public thus water is purified before drinking. In the study of chemistry, purification methods are studied.
Even after purification, a test of purity is significant so as to ascertain the success of the purification method. For liquid sample, boiling point is employed to ascertain the purity of a liquid sample. A pure liquid consists of a fixed boiling point that is; the boiling is for all time at a fixed and constant temperature given the external pressure is constant.
Methods of Boiling Point Determination:
Some of the methods have been developed for finding out the boiling point of liquids. In this, we shall take two methods. The first is employed for liquids with non poisonous vapors whereas the second is employed for liquids having poisonous vapors.
The simplest process is to insert a thermometer into a boiling liquid in an open glass vessel or container. The temperature should be read by the thermometer within the boiling liquid. This process can be employed for liquids whose vapored are not poisonous or offensive. The distillation process is recommended for liquids by poisonous or offensive vapor.
Fig: Simple distillations
The liquid is positioned in the flask. Whenever the flask is heated, the saturated vapor pressure of the liquid increases till it becomes equivalent to the pressure above the liquid. Boiling takes place and the thermometer records the temperature of the liquid or vapor mixture for liquids which are flammable, the heating must be done electrically or indirectly by employing a liquid bath. For liquids with boiling points lower than 100°C, a water or steam bath might be employed.
Fig: Determination of boiling point of alcohol
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